WASHINGTON — Senate Democrats on Friday blocked a measure to advance funding for the Department of Homeland Security, deepening a congressional standoff that has triggered mounting delays at major airports across the country as Transportation Security Administration workers continue reporting to work without pay.

Democrats withheld the votes necessary to move the spending bill toward final passage. Senate Democratic Leader Chuck Schumer of New York announced he would bring an alternative measure to the floor Saturday — one limited strictly to TSA funding — though that proposal is broadly expected to fail as well, with lawmakers convening in an unusual weekend session to contend with the crisis.

Negotiations behind closed doors gained urgency Friday as White House border czar Tom Homan held his second straight day of talks with a bipartisan group of senators. At the heart of the Democratic position are demands for sweeping changes to federal immigration enforcement practices, driven in part by the shooting deaths of Alex Pretti and Renee Good in Minneapolis.

Democratic lawmakers exited Friday's meeting with Homan without offering comment to reporters. Sen. Susan Collins, the Maine Republican who chairs the Senate Appropriations Committee, indicated the White House had sweetened its offer in a bid to break the impasse, though she declined to detail the specifics of the proposal.

"It's a very fair, reasonable offer," Collins said, adding she hoped the two sides would meet again Saturday. "But that depends on whether the Democrats come back with a response."

Senate Majority Leader John Thune of South Dakota said he detected "deal space" emerging from the White House discussions, though he also cast doubt on whether Democrats were genuinely committed to reaching any agreement that would restore full funding for Immigration and Customs Enforcement.

"This is a pox on everybody's house," Thune said. "You've got people standing in lines at the airports. This needs to be fixed. It needs to get resolved and there are good-faith efforts being made finally on all the relevant issues."

On the Senate floor, Schumer acknowledged the urgency of restoring TSA operations but rejected the Republican framework for doing so, which ties TSA funding to broader Homeland Security appropriations. Democrats are pressing to fund the agency on a standalone basis while separate talks over ICE funding continue.

"Tomorrow, America will see the matter crystal clear: which senators want to open up TSA, pay TSA workers, and end the chaos at our airports, and which senators are going to block TSA funding yet again," Schumer said.

The overwhelming majority of TSA employees are classified as essential personnel and have remained on the job throughout the funding lapse — but without compensation. Absenteeism has begun to climb at several major airports, producing longer screening lines and heightened frustration among travelers nationwide.

Among the policy changes Democrats are seeking as conditions for a funding deal: a requirement that ICE agents obtain a judicial warrant before forcibly entering private residences, mandatory display of identifying information on agent uniforms, and a prohibition on the use of masks during enforcement operations.

"The American people have had enough of this rogue agency. We need to rein it in. And we are negotiating right now over how to do that," said Sen. Patty Murray of Washington, the ranking Democrat on the Senate Appropriations Committee.

The Trump administration contends it has already agreed to a number of concessions, among them an expanded body-worn camera program — with carve-outs for undercover operations — and restrictions on civil enforcement activities near sensitive locations including hospitals, schools and houses of worship. Republicans have also pointed to President Trump's dismissal of Homeland Security Secretary Kristi Noem and his decision to place Homan in direct command of operations in Minneapolis as evidence of the administration's willingness to reform ICE's conduct.

With Congress set to depart for a two-week Easter recess at the end of the month, Thune made clear he was prepared to keep senators in Washington if the dispute remains unresolved.

"I can't see us taking a break if the government is still shut down," Thune said.